INTRODUCTION

Orthodontically induced inflammatory root resorption is an inevitable side effect of orthodontic treatment.^1,2 The idea that orthodontic tooth movement involves a local inflammatory process in the surrounding periodontium has been used to explain the coupling between root resorption and orthodontic tooth movement, hence the name orthodontically induced inflammatory root resorption (OIIRR).³

Skeletal open bite is among the most difficult malocclusions to treat.⁴ Miniscrew-supported maxillary posterior segment intrusion has been validated as an effective treatment modality for such cases,^5–7 with results comparable to those of surgical outcomes. However, the exact mechanotherapy and the associated side effects of posterior teeth intrusion, including root resorption, have not been thoroughly investigated.^2,8–10

The force magnitude needed for effective posterior dentoalveolar intrusion has been a subject of interest, in which various force magnitudes have been used, ranging from 150 g per segment^11,12 to 500 g per segment.^6,7 It has been reported that intrusive forces were associated with increased risk of root resorption,^9,10 because all applied force would be concentrated at the root apex, disturbing the capillary blood flow and initiating the resorptive process.^10,13 However, the fact that increasing the force magnitude used for intrusion would in turn increase the amount of root resorption has been debated. Some studies reported a direct relationship between the applied force magnitude and the resulting resorption,^14,15 while others refuted such a correlation.^16,17 A recent systematic review reported low-quality evidence to support the fact that intrusive forces with increased levels would cause increased risk of root resorption and recommended conducting well-designed trials to elucidate such findings.¹⁰

The aim of this study was to compare the effects of two different force magnitudes used for miniscrew-supported maxillary posterior dentoalveolar intrusion on the amount of root resorption of the intruded teeth.

MATERIALS AND METHODS

Participants, Eligibility Criteria, and Settings

This study was carried out in the clinic of the Orthodontic Department, Faculty of Dentistry, Cairo University, after approval by the Center of Evidence Based Dentistry and Research Ethics Committee. Informed consent was obtained from all participants. Inclusion and exclusion criteria are shown in Table 1.

Table 1. Patient Eligibility Criteria

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Interventions and Data Analysis

All patients received a segmented fixed appliance (Roth prescription, 0.022 × 0.028-inch slot, Gemini brackets, 3M Unitek, St Paul, Minn) on the maxillary posterior segments (from the first premolar to the second molar) bilaterally. Leveling and alignment were performed until reaching 0.019 × 0.0250 inch stainless steel segmented archwire, then intrusion was started using infra-zygomatic and palatal miniscrews (3M Unitek TAD, 10 × 1.6 mm). The comparison group involved applying 200 g of intrusive force to the posterior segments (equivalent to 20 g per root). For the intervention group, 400 g of intrusive force was applied to the same segments (equivalent to 40 g per root). A continuous intrusive force was applied using nickel-titanium coil springs for 6 months (Figure 1). Patients were recalled every 2 weeks to check the integrity of the appliance assembly and stability of the miniscrews. Recalibration of the coil springs was done every 4 weeks to ensure force continuity. Details of the force measurement and recalibration of the coil springs were discussed in detail in a previous publication.⁵

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A customized three-dimensional analysis was specifically created for this study using InVivo 5 (Anatomage) software version 5.3r (Anatomage, San Jose, Calif). Root resorption measurements were taken as root lengths (measured from the cusp tip to the root apex of each root of all posterior teeth on both sides) on the pre- and postintrusion cone-beam computed tomographic images (CBCTs). The postintrusion value was subtracted from the preintrusion length to calculate the length of the root that had been resorbed during the intrusion phase (Figure 2).

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After measurements of individual roots were made and recorded, the data were pooled for premolar and molar roots separately, and then this was pooled for all intruded roots.

RESULTS

Participant Flow

Two subjects (one in each group) dropped out of the study, leaving 20 subjects to be included in the analysis (Figure 3).

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Baseline Characteristics

Participants' age, amount of open bite, mandibular plane angle, and root lengths at baseline were gathered and compared to ensure similarity and therefore validate the comparison (Tables 2 and 3).

Table 2. Comparison at Baseline Between Intervention and Comparison Groups^a

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Table 3. Comparison of Baseline (Pretreatment) Root Lengths Between the Intervention and Comparison Groups^a

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Root Resorption Measurements

The resorption of all roots in both groups showed statistical significance, except for the buccal root of the right first premolar, palatal root of the left second premolar, and mesiobuccal root of the left second molar in the comparison group. The exceptions in the intervention group were the distobuccal and palatal roots of the right first molar. However, the pooled result showed high statistical significance for the premolar and molar roots as well as all roots in both groups (Table 4). The pooled resorption measurements were 0.84 ± 0.96 mm and 0.93 ± 1.0 mm for the comparison and intervention groups, respectively. On the other hand, comparative statistics (Table 5) showed no statistically significant difference between the groups for resorption measurements of individual roots as well as their pooled result (Figure 4).

Table 4. Descriptive Statistics for Root Resorption of Individual Roots for Both the Intervention and Comparison Groups^a

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Table 5. Comparative Statistics of Resorption of Individual Roots and the Pooled Results for Premolar, Molar, and All Roots^a

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The inter- and intraobserver measurements showed high concordance coefficients, confirming good reliability (Table 6). Exceptions to this were the distobuccal root of the first molar, which showed low interobserver reliability, and the palatal root of the same tooth, which showed low intraobserver reliability (Table 6).

Table 6. Intra- and Interobserver Reliability Measurements for Root Resorption^a

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DISCUSSION

Root resorption is considered one of the most common side effects of orthodontic treatment. It has been reported in association with different tooth movements and a wide variety of appliances.¹⁰ Since intrusive forces have been associated with causing increased amounts of root resorption, with heavier forces claimed to aggravate the incidence and extent of this event,^10,19,20 this study aimed to compare the effects of two different intrusive force magnitudes. Forces were applied to the maxillary buccal segments in the context of miniscrew-supported intrusion for treatment of skeletal open bite in adult patients.

Choosing the randomized clinical trial methodology, together with allocation concealment and blinding of the outcome assessor, were performed to enhance the reliability of the reported outcomes and reduce bias. This was especially important, as the available systematic reviews reported a lack of high-quality, methodologically sound studies that would enable conclusive evidence to be drawn.^8–10,20 In addition, the pre- and postintrusion measurements (ie, the availability of baseline information about root lengths in this study) increased the confidence that any change was due only to the intervention and not to any previous root shortening. This was also among the prerequisites of study inclusion in some systematic reviews.⁹

Individual root resorption measurements were made to provide a large amount of information, followed by pooling for premolar and molar teeth separately as well as for all roots within the same group. Such pooling aimed at simplifying the analysis and comparison with other studies. Generally, there was statistically significant root resorption within each group, with a mean of 0.84 ± 0.96 mm and 0.93 ± 1.00 mm for the comparison and intervention groups, respectively. However, the difference between the groups was neither statistically nor clinically significant. Therefore, the results of this study refute that the amount of resorption was closely related or proportional to the applied force magnitude. The lack of correlation between the magnitude of the applied force and the amount of root resorption was similar to the findings of Carillo et al.¹⁶ and Ramirez et al.,¹⁷ although it was different from that of Harris et al.¹⁴ and Gonzales et al.,¹⁵ who found a direct relationship between the magnitude of the applied force and the depth of resorption lacunae. Regarding the numbers, the findings of this study were inconsistent with those reported in a recent systematic review, which found no randomized clinical trials to include regarding measurements for posterior teeth intrusion and reported only a mean of 0.41 mm of resorption for these teeth.²⁰ The quality of evidence was rated as low, and measurements were made on periapical radiographs, panoramic radiographs, as well as CBCTs, all pooled together.²⁰

The amounts of resorption previously mentioned in the literature were variable. Some studies reported that root resorption due to intrusion was only minimal, measuring about 0.1 mm, using force magnitudes of 50 to 200 g.^16,21,22 However, these were experimental trials that cannot be applied to human teeth because of biological, physiological, and structural differences.²³ Heravi et al.²⁴ quantified root resorption after molar intrusion using skeletal anchorage to be 0.3, 0.4, and 0.6 mm for the palatal, distobuccal, and mesiobuccal roots, respectively, of the first molars. Their assessment was performed using intraoral radiographs, which may not have been very accurate, as they tend to underestimate the amount of resorption.²⁵ Ari-Dimarkaya et al.²⁶ found 0.02 to 2.49 mm of resorption in the group that had intrusive force to zygomatic miniplates as anchorage, which was 0.58 mm greater than the control group, but this difference was considered to be clinically insignificant. They assessed root resorption using panoramic radiographs, which were reported to suffer some inaccuracies; hence, their study was excluded from systematic reviews.⁹ On the other hand, CBCT used in the current study was reported to be more accurate in the detection of apical root resorption.^27–29

The definition of heavy forces varied between the current study and other reports. In the current study, heavy force was planned to be 40 g per root, which was reasonable to achieve efficient intrusion without risking an increase in the amount of root resorption. This is especially important, as this study was conducted in vivo on teeth that were not indicated for extraction. Other studies that reported the application of more than 200 g per root were either experimental or conducted on premolars indicated for extraction, which enabled the application of such heavy forces.²⁴

An interesting finding in the current study was the difference in the amounts of resorption found in different roots. In both groups, the first premolars had greater resorption than the second premolars did, followed by the first and second molars. This may have been due to the greater proximity of the premolars to the maxillary sinus floor as compared with the molars.³⁰ Such proximity may have offered greater resistance to intrusion and hence caused more resorption of the premolar roots. Similar findings were reported by Çifter and Saraç²⁹ in their finite element model, in which the apical areas of the first premolars and mesial roots of the first molars showed the highest stress concentrations, suggesting they might have been more prone to resorption.

The detection of root resorption in the current study presented some difficulty. This was obvious in the intra- and interobserver reliability measurements, especially for the distobuccal and palatal roots of the first molars. A similar finding was reported by Ari-Demarkaya²⁶ concerning the palatal root, which may have been due to its multidirectional curvature, which made it difficult to accurately locate the apex. However, this difficulty should be reduced in CBCT as compared with panoramic radiographs used in other studies.^24,26

Combining the results of this study with a previous publication,⁵ it can be noted that there was no statistically significant difference in the amount of posterior teeth intrusion or anterior open bite closure by using the 200-g and 400-g forces.⁵ Hence, it cannot be recommended to use one of the forces over the other according to the reported outcomes. However, lighter forces can generally be recommended for their claimed benefits for a favorable tissue reaction. In this study, no histological investigation could be performed since it was a human study on teeth that were not indicated for extraction.

CONCLUSIONS

• Root resorption is an inevitable outcome associated with maxillary buccal segment intrusion irrespective of the magnitude of the intrusion force used.

• No direct correlation could be found between the magnitude of the intrusive force and the amount of root resorption.

• The root resorption resulting from miniscrew-supported maxillary buccal segment intrusion is considered minimal and clinically insignificant compared with the benefits achieved from such a conservative treatment modality for skeletal open-bite patients.